With the increased computerization of documents, search technology has become more important for finding a desired document from a large amount of documents that have been accumulated so far. A typical search using an electronic device finds a document by including a search term inputted by the user from a document group to be searched, and displays the found document to the user.
In natural language, what a term means can be expressed by another term. For example, the term, “the largest island in the world” has the same meaning of the term, “an island that is the largest in the world”. In searching by simply finding the same string, if the user inputs “the largest island in the world” as a search term, the user cannot find any document that includes “an island that is the largest in the world”. Therefore, herein lies a problem in that it is difficult to find documents that are desired by the user. In order to solve this problem, various arts to extend a search term to an appropriate similar meaning term and search by the extended search keyword have been recently developed for improving the usability of search engine.
For example, Patent Literature 1 (Unexamined Japanese Patent Application KOKAI Publication No. 2004-118262) discloses an art to extend a search term to a synonym and also to reduce the number of documents to be searched in order to reduce the search noise. Patent Literature 2 (Unexamined Japanese Patent Application KOKAI Publication No. 2006-215717) discloses an art to evaluate the similarity between a synonym to a search term and to conduct the search based on the similarity, thereby preferentially searching by the similar meaning term that has more similar meaning to the search term.
Extending a search term to a synonym can decrease search omission, but does not necessarily lead to an effective search. For example, an unnecessary search may be performed, such as searching a document that the user does not intend to search or searching the same document by a plurality of synonyms. Therefore, there is a need for an art to extend the search term to a precise synonym and also to perform an effective search.